Radio frequency power measurement traceable to redefined SI units

In November, the International System of Units (SI) is expected to be redefined to reference fundamental physical constants as the foundation for units of mass, temperature, current and amount of a substance. This redefinition provides an opportunity to rethink accurate measurement methods and redesign them to be directly traceable to the new SI.

One such measurement is the power of an electromagnetic wave. The common method of quantifying power, which has not changed for more than 100 years, relies on measuring the equivalence between thermal absorption and electrical heating, referred to as DC substitution. The heating power that produces the same temperature increase as the original radio frequency (RF) or optical energy is taken to be the radiative power of the electromagnetic (EM) wave. Therefore, traceability to photonic optical power is established by way of electrical measurements. While this approach is SI traceable, the path is convoluted.

Holloway et al. describe a new method for measuring power realized by the redefined SI and Planck’s constant. The authors use a commercial microbalance to measure the force from a pulsed RF beam reflecting from a copper mirror. The mass displayed on the balance is proportional to the radiation pressure (or force) carried by the EM wave, which is proportional to power.

The accuracy of this early proof of concept was within 10 percent of the conventional method. The authors expect that with further development, this technique will be used to calibrate devices over a larger range of power and with much higher accuracy.

Source: “Using radiation pressure to develop a radio-frequency power measurement technique traceable to the redefined SI,” by Christopher L. Holloway, Matthew T. Simons, Marcus D. Kautz, Abdulaziz H. Haddab, David Novotny, John H. Lehman, Paul A. Williams, and Gordon A. Shaw, Applied Physics Letters (2018). The article can be accessed at https://doi.org/10.1063/1.5052258 .